Stunnel is used to provide secure encrypted connections for clients or servers that do not speak TLS or SSL natively.[4] It runs on a variety of operating systems,[5] including most Unix-like operating systems and Windows. Stunnel relies on the OpenSSLlibrary to implement the underlying TLS or SSL protocol.
Stunnel is maintained by Polish programmer Michał Trojnara and released under the terms of the GNU General Public License (GPL) with OpenSSL exception.[7]
Example
A stunnel can be used to provide a secure SSL connection to an existing non-SSL-aware SMTP mail server. Assuming the SMTP server expects TCP connections on port 25, the stunnel would be configured to map the SSL port 465 to non-SSL port 25. A mail client connects via SSL to port 465. Network traffic from the client initially passes over SSL to the stunnel application, which transparently encrypts and decrypts traffic and forwards unsecured traffic to port 25 locally. The mail server sees a non-SSL mail client.[citation needed]
The stunnel process could be running on the same or a different server from the unsecured mail application; however, both machines would typically be behind a firewall on a secure internal network (so that an intruder could not make its own unsecured connection directly to port 25).[citation needed]
